1. Field of the Invention
The present invention relates generally to satellite attitude control and more particularly to a method and apparatus for using magnetic torquers to perform reaction wheel desaturation without the need for magnetometers or magnetic field models.
2. Description of Related Art
Spacecraft attitude control is required to maintain proper orientation of the spacecraft relative to earth. To maintain a desired position, the attitude control system must account for the earth's movement and counteract both internal and external disturbances that cause undesired attitudinal movement of the craft.
The use of reaction or momentum wheels to effect precision attitude control is well known in the art. The reaction wheels are positioned within and rotate independently of the spacecraft. This rotation causes a torque to act on the inertial mass of the spacecraft, thereby inducing a radial acceleration that causes attitudinal movement of the craft. In this way the torque created by the reaction wheels is used to effect attitude control.
Reaction wheels are especially effective at offsetting internal disturbances that are typically periodic in nature and are due to mechanical device movement or vibration onboard the spacecraft. However, reaction wheels acting alone are not sufficient to counteract the external disturbance torques that are typically non-periodic in nature and result from phenomenon such as nearby space debris, passing comets, etc. To offset the effect of these disturbances, magnetic torquers are often employed to limit the speed at which the reaction wheels rotate, an operation that is referred to in the art as wheel desaturation. By creating a torque that opposes the wheel torque, the magnetic torquers cause a decrease in the wheel speed. In the simplest form, the torquers are solenoids that are energized to cause current flow therein. When energized, a magnetic field is induced having an intensity, H, given by the following expression: ##EQU1##
where PA1 L is the length of the current path, PA1 n is the number of turns in the solenoid, PA1 i is the current; and PA1 a.sub.H represents a unit vector having a direction that is perpendicular to the direction of the current flow within the solenoid.
This resulting magnetic field seeks to align itself with the earth's magnetic field causing a torque to act on the body of the craft. Since the direction of the induced magnetic field, H, is perpendicular to the direction of the current flow, the polarity of the power source applied to the coil determines the direction of the induced field. Once the direction and strength of the earth's magnetic field is known, a power source of proper polarity applied to the solenoid induces a magnetic field that produces the desired torque. Thus, the key to creating a desired torque is determining the characteristics of the earth's magnetic field with respect to the position of the craft.
Conventional systems use magnetometers or magnetic field models to determine the earth's magnetic field with respect to the spacecraft. However, determination of the earth's field using magnetometers involves measurement of the earth's field requiring a full complement of weight-intensive magnetic field sensors. Magnetic field models that allow estimation of the earth's magnetic field given the position of the craft with respect to the earth and sun are also disadvantageous because the estimation performed with the magnetic field model requires complex circuitry that increases the complexity of the spacecraft design. The added weight and design complexity inherent in using either magnetometers or magnetic field models increase the cost and energy consumption of the spacecraft. Therefore, it is desirable to have a method and apparatus for performing magnetic torquer control without the additional weight, cost, power consumption and operational complexity ordinarily associated with magnetometers and/or magnetic field models.